[NO-JIRA][BpkModal] Add custom padding element to scrim util and modal

[ss-tombrown]

This PR adds support to specify a custom padding element to Backpack Modal.

When a modal is opened, padding is added to the body to prevent the page from horizontally shifting. However, some microsites (like Banana) are setup in a way that has no effect, for example when using an absolutely positioned content.

This addition allows passing in a custom element instead, so that the padding can be applied to the correct element, instead of the body.

Remember to include the following changes:

• Ensure the PR title includes the name of the component you are changing so it's clear in the release notes for consumers of the changes in the version e.g [KOA-123][BpkButton] Updating the colour
• README.md (If you have created a new component)
• Component README.md
• Tests
• Accessibility tests
  • The following checks were performed:
    • Ability to navigate using a keyboard only
    • Zoom functionality (Deque University explanation):
      • The page SHOULD be functional AND readable when only the text is magnified to 200% of its initial size
      • Pages must reflow as zoom increases up to 400% so that content continues to be presented in only one column i.e. Content MUST NOT require scrolling in two directions (both vertically and horizontally)
    • Ability to navigate using a screen reader only
• Storybook examples created/updated
• For breaking changes or deprecating components/properties, migration guides added to the description of the PR. If the guide has large changes, consider creating a new Markdown page inside the component's docs folder and link it here

[github-actions]

Visit https://backpack.github.io/storybook-prs/3696 to see this build running in a browser.

[github-actions]

	Warnings
⚠️	Package source files (e.g. packages/package-name/src/Component.js) were updated, but snapshots weren't. Have you checked that the tests still pass?
⚠️	Package source files (e.g. packages/package-name/src/Component.tsx) were updated, but type files weren't. Have you checked that no types have changed?

Browser support

If this is a visual change, make sure you've tested it in multiple browsers.

Generated by 🚫 dangerJS against a70627b

[github-actions]

Visit https://backpack.github.io/storybook-prs/3696 to see this build running in a browser.

[github-actions]

Visit https://backpack.github.io/storybook-prs/3696 to see this build running in a browser.

[amburrrshand]

Looks good to me!